In conventional hybrid electric vehicle driving devices, such as that described in Patent Literature 1 for example, a motor casing that holds a stator internally is integrated by being joined to a housing of a transmission so as to have a joint end portion of an intermediate housing that is coupled to the engine interposed when fitted inside the intermediate housing. A first axial end of the motor casing is formed into a large diameter portion, a second axial end is formed into a small diameter portion, and O-ring housing grooves are respectively formed on outer circumferential surfaces of the large diameter portion and the small diameter portion. O-rings are housed in the O-ring housing grooves and are pressed and held between the motor casing and the intermediate housing, and a coolant channel is formed between the motor casing and the intermediate housing.